In phase change optical discs (phase change optical recording media) such as CD-RW, generally a basic constitution is made by providing a recording layer composed of a phase change material on a plastic substrate and thereon forming a reflection layer which enhances an optical absorption ratio of the recording layer and has a thermal diffusion effect, and the information is recorded and replayed by irradiating the laser light from a substrate side.
Phase change recording materials exhibit the phase change between a crystalline state and an amorphous state by heating with laser light irradiation followed by cooling. When rapidly cooled after rapid heating, they become amorphous whereas when slowly cooled, they are crystallized. The phase change recording medium practically applies this nature to record and replay of the information.
Additionally, for the purpose of blocking oxidation, transpiration and deformation of the recording layer which occur by heating with light irradiation, typically a lower protection layer is provided between the substrate and the recording layer, and an upper protection layer is provided between the recording layer and the reflection layer. These protection layers have regulation functions of optical properties of the optical recording medium, and the lower protection layer also combines with the function to prevent the substrate from softening due to the heat when recorded in the recording layer.
In recent years, along with the increase of information amounts covered by computers, signal recording capacities of rewritable type discs such as DVD-RAM, DVD-RW and DVD+RW have been increased, and density growth of signal information has been advanced. The recording capacity of current CD is about 650 MB, that of DVD is about 4.7 GB, and it is forecasted that the density growth of the recording is highly required in the future. Along with the increase of the information amounts, it appears to also require enhancement of a recording speed. At present, as the rewritable DVD disc, those capable of recording 8 times faster in a monolayer have been developed and practically applied.
As methods to increase the recording density using such a phase change optical recording medium, for example, shortening the laser wavelength used to a blue color area, or enlarging a numeric aperture NA of an objective lens used for pickup which performs record and replay to reduce a spot size of the laser light irradiated to the optical recording medium has been proposed, researched, developed and almost come into practical use.
As the method to increase the recording capacity by improving the optical recording medium itself, various types of dual layer phase change optical recording media made by overlapping two information layers composed of at least the recording layer and the reflection layer at one side of the substrate and adhering these information layers with ultraviolet ray-curable resins have been proposed. A separation layer (sometimes referred to as an intermediate layer) which is an adhered portion between these information layers has the function to optically separate two information layers, and made up of a material which does not absorb the laser light as possible because it is necessary that the laser light used for the record and replay attains abundantly as possible to the information layer at an inner side.
There are still many problems for this dual layer phase change optical recording medium.
For example, if the laser light is not sufficiently transmitted through the information layer (first information layer) at a front side as seen from the side of laser light irradiation, the information can not be recorded in the recording layer of the information layer (second information layer) at the inner side and can not be replayed. Thus, the reflection layer which constitutes the first information layer must be an ultrathin translucent reflection layer. To replay, a high reflectance ratio as possible is required. However, in the optical recording medium having two layers or more of the recording layers and the reflection layers, their optical absorption and optical transmission affect, and the reflectance ratio of the optical recording medium itself becomes low.
Recording in the phase change optical recording medium is performed by irradiating the laser light for a very short time to the phase change material in the recording layer followed by rapid cooling to change the crystal to the amorphous and form a mark. Thus, in the case of the translucent reflection layer with a very thin thickness of about 10 nm, a radiator effect become excessively small compared with the optical recording medium having the monolayer. Therefore, it becomes difficult to form amorphous marks when recorded in the recording layer of the first information layer. As a result, a modulation degree is hardly assured.
In Patent Document 1, the technology in which materials selected from AlN, Al2O3, Si3N4, SiO2, Ta2O5, TaO, ZrO2, ZnO, TiO2, SiC and composite materials thereof are used for a transparent heat release layer (sometimes referred to as an optical transmission layer), and ZnS—SiO2 is used for a transparent dielectric layer (sometimes referred to as a protection layer) has been disclosed, but no special effect of Sn oxide is described.
In Patent Document 2, a multilayer phase change information recording medium in which a thermal diffusion layer (sometimes referred to as an optical transmission layer) contains tin oxide as a major component and at least antimony oxide has been disclosed, but it is not described that the upper protection layer and the thermal diffusion layer, or the lower protection layer and the upper protection layer and the thermal diffusion layer are composed of Sn oxide. The effect in the blue color laser wavelength area at 405 nm is emphasized, and is different from the present invention.
Furthermore in Patent Document 3, a multilayer phase change information recording medium in which the thermal diffusion layer contains ITO (indium oxide-tin oxide) as the major component and at least one of Al and Ga has been disclosed, but it is not described that the upper protection layer and the thermal diffusion layer, or the lower protection layer and the upper protection layer and the thermal diffusion layer are composed of Sn oxide. The effect in the blue color laser wavelength area at 405 nm is emphasized, only the special effect of using ITO for the thermal diffusion layer is described, and this is different from the present invention in constitution.    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2002-298433    Patent Document 2: JP-A No. 2004-47038    Patent Document 3: JP-A No. 2004-47034